Release Area W16-8 is located in the Bedout Sub-basin of the Roebuck Basin and extends onto the Broome Platform and Willara Sub-basin, in the offshore Canning Basin, Western Australia (Figure 1). The Release Area lies 120 km from the northwest coast of Western Australia in water depths of 85−160 m. The sub-basin is a frontier region with only 8 wells, including discoveries of oil at Phoenix South 1 and liquids-rich gas at Roc 1. No wells have been drilled in the offshore Broome Platform or the Willara Sub-basin.

The Bedout Sub-basin consists of an east-northeast to west-southwest-trending depocentre (Figure 1) filled with approximately 2.5 km of Paleozoic and 7 km of Mesozoic sediments (Smith et al, 1999). It is separated from the Beagle Sub-basin of the Northern Carnarvon Basin to the west by the North Turtle Hinge Zone, and is partly bounded to the northwest by the Bedout High. The Mesozoic succession has generally experienced only mild structuring, and thickens to the west before pinching out against, and partly draping over, the Bedout High. To the east and south the Mesozoic sediments thin and progressively onlap the older Paleozoic succession.

The Broome Platform is an uplifted area of shallow pre-Ordovician rocks (Figure 1), capped by a thin succession of Ordovician, Devonian and Permian rocks (approximately 1−2 km thick) that dip gently to the southeast (Figure 2) (Kennard et al, 1994). The offshore part of the Willara Sub-basin (Figure 1) and flanking terraces also contain a Paleozoic sedimentary succession, thought to be approximately 2 km thick based on sparse, poor quality, seismic data, although its maximum thickness is unknown. This depocentre has not been drilled, but is inferred to contain a similar succession to that intersected onshore, where the Willara Sub-basin contains an Ordovician–Devonian succession that accumulated in a series of half graben, unconformably overlain by Permian Grant Group glacial sediments (Romine et al, 1994). The Paleozoic rocks are, in turn, unconformably overlain by a Jurassic–Cretaceous succession that thickens offshore (Totterdell et al, 2014).

An active petroleum system in the Release Area is suggested by the discoveries at Roc 1 and Phoenix South 1. The presence of oil has challenged the interpretation of a predominantly gas-prone petroleum system and led to the suggestion that the Phoenix 1 well, drilled in 1980 and originally classified as a gas discovery, may actually contain significant gross oil pay. Modelling of potential Rowley and Bedout Sub-basin source rocks indicates that liquid hydrocarbons are currently being generated.

Willara Sub-basin

Possible reservoirs include the Nita Formation, Acacia Sandstone Member of the Willara Formation and other carbonates.

Seals

Seals may include the Carribuddy Group shales and carbonates.

Traps

Possible anticlinal closures.

Infrastructure and markets

No infrastructure is present or currently planned within the Roebuck Basin, but given recent exploration successes, future development is being considered. The nearest oil infrastructure is approximately 200 km to the southwest where the Mutineer-Exeter oil accumulation (Northern Carnarvon Basin) is serviced by the Modec Venture 11 floating production, storage and offloading (FPSO) facility. Fletcher-Finucane is also tied back to the Mutineer-Exeter FPSO. Gas pipelines are in operation approximately 220 km to the southwest, servicing the Angel gas accumulation.

Critical risks

The presence of an Ordovician succession in the offshore Canning is inferred from correlation to the onshore basin. If present, it is potentially overmature in the Oobagooma Sub-basin, and under-mature in the offshore Willara Sub-basin.

Limited and variable quality seismic data (Figure 3) indicate that the Willara Sub-basin succession encountered onshore is likely to continue in to the offshore parts of the sub-basin; however, no wells have been drilled to test for the presence of any petroleum systems.

The Bedout Sub-basin is in a frontier basin and until the recent discoveries at Phoenix South 1 and Roc 1, the lack of a proven petroleum system was the major risk. The paucity of exploration wells in the sub-basin has resulted in a poor understanding of the petroleum systems, migration pathways and play types (Figure 2) in the region. There is relatively limited and variable quality seismic data (Figure 3) and no wells in the release area. The closest wells to the Release Area are Bedout 1 (22 km west), La Grange 1 (30 km west) and Keraudren 1 (50 km southwest). The area was initially thought to be predominantly gas-prone due to the misinterpreted gas discovery at Phoenix 1 (which is now believed to possess a significant oil column). However, evidence of palaeo-oil columns exist within the basin and the recent discoveries indicate that the Roebuck Basin is both oil- and gas-prone. The main reason for the lack of discoveries in this basin was originally thought to be the lack of valid traps caused by the relatively gentle structuring in the basin rather than a lack of hydrocarbon charge. However, the recent dry exploration wells in the outer Rowley Sub-basin (Hannover South 1, Steel Dragon 1 and Anhalt 1) indicate that lack of source rocks is the critical risk in that area. The relatively low number of discoveries may be simply due to the small number of wells, and despite this paucity, a relatively good success rate of 3 (including Phoenix 1) out of 14 wells (>21%), makes the Roebuck Basin deserving of further exploration. The recent discoveries and the planned appraisal and exploration wells (e.g. Roc 2), will further de-risk the Roebuck Basin.

Marine and environmental information

Release Area W16-8 characteristics

located over slope, terrace and shelf geomorphic features in water depths of approximately 85 160 m, 120 km from the nearest coast to the southeast, at Eighty Mile Beach.

Seabed sediment is generally gravelly sand in texture, and carbonate in composition Baker et al, 2008. A large proportion (25-50 %) of sediment is relict James et al, 2004, and in places is sediment starved.

No information is available for the shallow (< 100 m) subsurface geology

Potential hazards include tropical cyclones and internal waves. The area is also affected by the Leeuwin Current

LIPSKI, P., 1993—Tectonic setting, stratigraphy and hydrocarbon potential of the Bedout Sub-basin, North West Shelf. The APEA Journal, 33(1), 138–150.

LIPSKI, P., 1994—Structural framework and depositional history of the Bedout and Rowley sub-basins. In: Purcell, P.G. and Purcell, R.R. (editors), The Sedimentary Basins of Western Australia. Proceedings of the West Australian Basins Symposium, Petroleum Exploration Society of Australia, Perth, 14–17 August, Perth, 769–777.

The department acknowledges the traditional owners of the country throughout Australia and their continuing connection to land, sea and community. We pay our respect to them and their cultures and to the elders past and present.